Sodium and magnesium are important cations found in atmospheric aerosols. Their salts of chlorides are hygroscopic. In this research, the mixed aqueous electrolyte system of magnesium and sodium chlorides has been studied with the hygrometric method at the temperature 298.15 K. The relative humidities of this system are measured at total ionic strength from 0.45 to 6 mol kg(-1) for different ionic strength fractions y of NaCl with y = 0.33, 0.50, and 0.67. The obtained data allow the deduction of water activities and osmotic coefficients. The experimental results are compared with the predictions of our extended composed additivity (ECA) law proposed in our previous work, the Zdanovskii-Stokes-Robinson (ZSR), the Robinson-Stokes (RS), the Reilly-Wood-Robinson (RWR), the Pitzer, and the Leitzke-Stoughton (LS II) models. Predictions made using these models are, in general, consistent with our results. From these measurements, new Pitzer mixed ionic parameters are determined and used to predict the solute activity coefficients in the mixture at total ionic strength for different ionic strength fractions.